Abstract:

The Next Generation Space Telescope (NGST) will perform wide-field
imaging in the high-radiation environment of deep space. Outside any
protective magnetic fields, cosmic rays will produce many
charged-particle events on the imaging detector, affecting as much as
10% of the field-of-view during a baseline 1000-second obervation.
We present an algorithm that identifies cosmic ray events in a series
of non-destructive readouts during an observation. Test results, in
which over 99% of of the cosmic rays are identified and removed
without significant degradation of the accumulated image data, are
also presented.

The Next Generation Space Telescope (NGST), planned for launch in
2007, will be a large aperture, passively-cooled observatory
concentrating on the infrared (IR) part of the spectrum. One of the
primary missions of the NGST is to provide infrared deep images of
distant (high-redshift) galaxies. As of this writing, the project
team's goal is to push the NGST observation bandpass to wavelengths as
long as 10 microns and beyond. To do this, the telescope's mirror and
optical assembly must be kept extremely cold--on the order of 40 K.
In order to achieve this low temperature without expensive and massive
active cooling equipment, the telescope must be located in deep space.
Therefore, the NGST is planned to be launched into an L2 halo orbit,
about 1 million miles from the Earth.

This puts the NGST beyond the protective influence of any planetary
magnetic field, exposing it to a large number of cosmic rays. When a
cosmic ray passes through the detector, its charge will falsely
trigger the detector, ruining the data in that part of the detector.
During a baseline 1000-second exposure, we anticipate that 10% of the
detector will be hit by a cosmic ray. This high level of data loss
will significantly impact NGST's science return.

2. Rejecting Cosmic Rays

By ``rejecting'' cosmic rays, we are referring to techniques to
digitally analyze image data and identify and discard cosmic
ray events in the detector, preserving only clean data. We can accomplish
this by reading the detector frequently and then identifying cosmic rays as
unusual events on the detector.

The NGST detectors will have non-destructive read-out capability.
Therefore, we can read the detector multiple times during an
observation while keeping the integrated observation data. The
detector is assumed to have a 16-bit dynamic range and an intrinsic
read noise uncertainty of electron units. We
unrealistically assume, for now, that there are no systematic errors
and that we know the dark field and flat field completely. (We plan to
add non-trivial dark field and flat field components in future tests.)

Our cosmic ray identification algorithm involves performing 65
non-destructive reads on the detector. We designate these values
S0, S1 and so on up to S64. S0 is performed at
the start of the sequence (t=0) and the remaining reads are
distributed evenly throughout the 1000-second observation (i.e.
ti
= i * 1000/64). For the purpose of identifying cosmic rays, we
compute the differences
D0 ... D63, where
Di = Si+1 -
Si. We then identify cosmic rays events as being instances where
Abs(Di - Median(Di)) > 5 * AbsDev(Di). We use the median
and absolute deviation (instead of the more traditional mean and
standard deviation) because the former are more robust when the data
sample contains outliers (Press et al. 1986). In particular, we
discovered that the median and standard deviation failed when a data
sample was impacted by multiple cosmic rays (0.6% of the detector
will be impacted by multiple cosmic rays). We use the absolute value of
the difference to avoid biasing the data; since a few data reads
(>1 in 104) will randomly lie more than 5 deviations from the median, we must be careful to discard all of the ``natural'' outliers in both directions from the mean as well as the cosmic rays.

If a cosmic ray is detected in interval j, we repeat this algorithm
for
D0 ... Dj-1 and for
Dj+1 ... D63, and so on until
no cosmic ray candidates are found. Since we are looking for events
at the 5-deviation level, we expect that fewer than 1 ``good'' data
value in 104 data points will be rejected falsely.

After rejecting cosmic rays, we have a series of data read values
S0...S64 and a list of zero or more outliers from the mean for
the series, the vast majority of which should be cosmic ray events on
the detector. To calculate the value of the flux of the object, we
apply the optimum slope-fitting routine to the up-the-ramp data,
discarding the outliers. (We are concerned only with the slope--the
increase in detector counts over time. The zero-point of the
line is not needed for this calculation.)

We use a variant on the linear least-squares fit for each segment of
consecutive data reads that are not impacted by cosmic rays.
We compute a covariance matrix for the data values Mi,j, which is a
tridiagonal matrix where
, ( is the readout noise, electron units),
and Mi,j = 0 where
. We then compute
Ci,j = Mi,j-1, and find the slope of the line
.

The slope A is computed for each line segment that is not
interrupted by a cosmic ray event. Multiple slopes are then combined
into a single value using
, where Aj is the slope of line segment j and
Nj is the number of data reads making up segment j.

As seen from Fig. 1, the cosmic ray rejection algorithm
removes most of the cosmic ray events from the detector. 114319 cosmic
ray events occured on the detector during the 1000-second
observation shown here. 1669 cosmic rays, just over 1% of the total,
survived the detection and removal process.

If left unremoved, the cosmic ray events in the detector completely
ruined 10.3% of the image. The cosmic ray removal process left 1.4%
of the image as completely lost, and, because it threw out data reads,
reduced the signal-to-noise in 10.2% of the image (including false-positive
identifications). The algorithm is still in need of refinement; there were
606 pixels falsely identified as being impacted by cosmic rays along with
the 1669 surviving cosmic rays.

Due to financial and communications restrictions, we expect that
cosmic ray rejection may have to be done on-board the NGST. We expect
the NGST data downlink to be approximately
1.6 x 106 bits per
second for 8 hours per day for a total of 5.35GB per day. The near-IR
detector alone will contain 64 million 16-bit pixels, for a raw data
content of 128MB per data read. For 80 1000-second observations per
day, the near-IR camera alone will produce 10GB per day. This
requires a data compression ratio of a factor of 2 (Nieto-Santisteban
et al. 1999). However, if we wished to perform cosmic ray rejection
after downlink, the data from all 65 data reads would have to be
downlinked. This would require a communication rate of more than 600
GB per day, requiring a compression ratio of over 100.

We can identify and remove 99% of cosmic ray events on the NGST detector.
While there is room for improvement, the data quality and information content
are largely preserved by this algorithm.
This cosmic ray rejection method provides a way to combine 65 images
into one, and contributes a significant amount of data compression. This,
in turn, loosens one limiting factor on the NIR camera size due to
downlink capability. However, this will require significant computer
resources to properly handle a full 8k x 8k pixel detector.

Acknowledgments

These studies are supported by the NASA Remote Exploration and
Experimentation Project (REE), which is administered at the Jet
Propulsion Laboratory under Dr. Robert Ferraro, Project Manager.